THANK YOU THANK YOU!!!!
SIP ALG was my problem too (after 4hrs of trying various settings).
Brilliant to bring the topic up and solving the problem.
I have mine as g711u and t38 , not sure which one was used
Will play around with other voip provider too (so far success with call centric, wanted to use the T38 feature...how do I know when T38 is being used?)